Common methods of forming synthetic resin bodies from hardenable materials may make use of liquid or flowable streams of synthetic resins which harden within a mold cavity and are introduced by injection under pressure or are simply cast into the mold. Various procedures have been proposed for the different types of resin which are available and, for the purposes of this application, we designate as "hardenable" resin any of the synthetic resins commercially in use which have a fluid state upon introduction into the mold cavity and can be cooled or permitted to set or cure to provide a somewhat more rigid state even though the latter condition may not make the body fully stiff or rigid. This resin may include thermoplastics which can be plasticized or masticated and/or heated with or without application of pressure, to produce the liquid state and which harden or set upon cooling. The designation also includes thermosetting materials or duromers which harden in the presence of heat by chemical reaction and even such settable compositions which can cure at low temperatures to form a more or less rigid body. We also use the term "foamable" herein to describe synthetic resins which may expand within a mold cavity to produce a cellular, porous or reticulate structure by evolution of gases either as the liquid resin expands into the free space of the mold cavity or by gases which are produced by chemical reaction or vaporization. Foamable synthetic resins may be considered as having set or become rigid when they cease to expand and resist deformation although they may remain somewhat elastic or resilient.
In prior-art systems for molding synthetic resins of the above type and especially so-called structure foams, various techniques have been used. In one such system, the prepolymerized synthetic resin material, containing a foaming agent, is introduced into a mold cavity and permitted to expand. In other systems a foamable fluid resin is similarly introduced into the mold cavity and can expand or set therein. Frequently the synthetic resin bodies produced in molds of this type have a thin but dense skin or surface film where the synthetic resin material contacts the walls of the mold cavity. The result is a sandwich structure in which a relatively low-density core of the synthetic resin material is flanked between relatively dense but thin skins of the same material.
It is also known to provide a sandwich-type structures of synthetic resin material by expanding-mold systems in which the outer layers are formed in the mold cavity and the foamable material is injected between the nonfoaming dense or surface layer to spread the latter apart in the manner of the walls of a balloon. In yet another expanding-mold arrangement, the foamable material is injected or otherwise introduced into a mold cavity, is expanded to fill the latter and permitted to set, and the mold cavity is then enlarged to accommodate the second material. The latter then fills the space between the retreating-mold wall and the surface of the foamed body originally formed thereagainst. Such systems are employed whenever the characteristics of two or more synthetic resin materials are desired. Thus where the surface textures or physical properties of one synthetic resin and the core properties of foamable resin are sesigned, recourse may be had to these techniques. The techniques are also employed where different color effects are desired, one resin having one color while the other resin has another. All of these systems have, however, the disadvantage that the expanding mold requires complicated equipment and control devices and production of bodies in such mold cannot be readily carried out in an automatic or serial manner. Furthermore, the surface characteristics of the bodies are not always satisfactory. Finally, these systems have the disadvantage that the previously introduced synthetic resin material must fully set before the mold is expanded if the processes are to be reproduceable and the body must have a particular configuration and characteristic. This increases the production time per unit, in any case, to a prohibitive extent.